tty: rocket: reduce stack usage

The build of xtensa allmodconfig gives warning of:
In function 'get_ports.isra.0':
warning: the frame size of 1040 bytes is larger than 1024 bytes

Signed-off-by: Sudip Mukherjee <sudipm.mukherjee@gmail.com>
Acked-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20191018161712.27807-1-sudipm.mukherjee@gmail.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>